631 research outputs found
Intersubband transitions in nonpolar GaN/Al(Ga)N heterostructures in the short and mid-wavelength infrared regions
This paper assesses nonpolar m- and a-plane GaN/Al(Ga)N multi-quantum-wells
grown on bulk GaN for intersubband optoelectronics in the short- and
mid-wavelength infrared ranges. The characterization results are compared to
those for reference samples grown on the polar c-plane, and are verified by
self-consistent Schr\"odinger-Poisson calculations. The best results in terms
of mosaicity, surface roughness, photoluminescence linewidth and intensity, as
well as intersubband absorption are obtained from m-plane structures, which
display room-temperature intersubband absorption in the range from 1.5 to 2.9
um. Based on these results, a series of m-plane GaN/AlGaN multi-quantum-wells
were designed to determine the accessible spectral range in the mid-infrared.
These samples exhibit tunable room-temperature intersubband absorption from 4.0
to 5.8 um, the long-wavelength limit being set by the absorption associated
with the second order of the Reststrahlen band in the GaN substrates
Ferromagnetic (Ga,Mn)N epilayers versus antiferromagnetic GaMnN clusters
Mn-doped wurtzite GaN epilayers have been grown by nitrogen plasma-assisted
molecular beam epitaxy. Correlated SIMS, structural and magnetic measurements
show that the incorporation of Mn strongly depends on the conditions of the
growth. Hysteresis loops which persist at high temperature do not appear to be
correlated to the presence of Mn. Samples with up to 2% Mn are purely
substitutional GaMnN epilayers, and exhibit paramagnetic
properties. At higher Mn contents, precipitates are formed which are identified
as GaMnN clusters by x-ray diffraction and absorption: this induces a
decrease of the paramagnetic magnetisation. Samples co-doped with enough Mg
exhibit a new feature: a ferromagnetic component is observed up to
K, which cannot be related to superparamagnetism of unresolved magnetic
precipitates.Comment: Revised versio
Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns
We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films
grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures
(Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and
11 %. Their crystalline structure, morphology and composition have been
investigated by transmission electron microscopy (TEM), electron energy loss
spectroscopy and x-ray diffraction. In the whole range of growth temperatures
and Mn concentrations, we observed the formation of manganese rich
nanostructures embedded in a nearly pure germanium matrix. Growth temperature
mostly determines the structural properties of Mn-rich nanostructures. For low
growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal
decomposition resulting in the formation of vertical one-dimensional
nanostructures (nanocolumns). Moreover we show in this paper the influence of
growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns
size and density. For temperatures higher than 180deg C, we observed the
formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns
and nanoclusters coexist. Combining high resolution TEM and superconducting
quantum interference device magnetometry, we could evidence at least four
different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn
atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc
nanocolumns (120 K 400 K) and
(iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte
Polarity determination in ZnSe nanowires by HAADF STEM
High angle annular dark field scanning transmission electron microscopy is
used to analyze the polarity of ZnSe nanowires grown, by molecular beam
epitaxy, on GaAs substrates. The experimental results are compared to simulated
images in order to verify possible experimental artefacts. In this work we show
that for this type of nano-objects, a residual tilt of the specimen below 15
mrad, away from the crystallographic zone axis does not impair the
interpretation of the experimental images
A Pair of Dopamine Neurons Target the D1-Like Dopamine Receptor DopR in the Central Complex to Promote Ethanol-Stimulated Locomotion in Drosophila
Dopamine is a mediator of the stimulant properties of drugs of abuse, including ethanol, in mammals and in the fruit fly Drosophila. The neural substrates for the stimulant actions of ethanol in flies are not known. We show that a subset of dopamine neurons and their targets, through the action of the D1-like dopamine receptor DopR, promote locomotor activation in response to acute ethanol exposure. A bilateral pair of dopaminergic neurons in the fly brain mediates the enhanced locomotor activity induced by ethanol exposure, and promotes locomotion when directly activated. These neurons project to the central complex ellipsoid body, a structure implicated in regulating motor behaviors. Ellipsoid body neurons are required for ethanol-induced locomotor activity and they express DopR. Elimination of DopR blunts the locomotor activating effects of ethanol, and this behavior can be restored by selective expression of DopR in the ellipsoid body. These data tie the activity of defined dopamine neurons to D1-like DopR-expressing neurons to form a neural circuit that governs acute responding to ethanol
The Homeodomain Derived Peptide Penetratin Induces Curvature of Fluid Membrane Domains
BACKGROUND:Protein membrane transduction domains that are able to cross the plasma membrane are present in several transcription factors, such as the homeodomain proteins and the viral proteins such as Tat of HIV-1. Their discovery resulted in both new concepts on the cell communication during development, and the conception of cell penetrating peptide vectors for internalisation of active molecules into cells. A promising cell penetrating peptide is Penetratin, which crosses the cell membranes by a receptor and metabolic energy-independent mechanism. Recent works have claimed that Penetratin and similar peptides are internalized by endocytosis, but other endocytosis-independent mechanisms have been proposed. Endosomes or plasma membranes crossing mechanisms are not well understood. Previously, we have shown that basic peptides induce membrane invaginations suggesting a new mechanism for uptake, "physical endocytosis". METHODOLOGY/PRINCIPAL FINDINGS:Herein, we investigate the role of membrane lipid phases on Penetratin induced membrane deformations (liquid ordered such as in "raft" microdomains versus disordered fluid "non-raft" domains) in membrane models. Experimental data show that zwitterionic lipid headgroups take part in the interaction with Penetratin suggesting that the external leaflet lipids of cells plasma membrane are competent for peptide interaction in the absence of net negative charges. NMR and X-ray diffraction data show that the membrane perturbations (tubulation and vesiculation) are associated with an increase in membrane negative curvature. These effects on curvature were observed in the liquid disordered but not in the liquid ordered (raft-like) membrane domains. CONCLUSIONS/SIGNIFICANCE:The better understanding of the internalisation mechanisms of protein transduction domains will help both the understanding of the mechanisms of cell communication and the development of potential therapeutic molecular vectors. Here we showed that the membrane targets for these molecules are preferentially the fluid membrane domains and that the mechanism involves the induction of membrane negative curvature. Consequences on cellular uptake are discussed
Dosage effect on uropathogenic Escherichia coli anti-adhesion activity in urine following consumption of cranberry powder standardized for proanthocyanidin content: a multicentric randomized double blind study
International audienc
Propolis can potentialise the anti-adhesion activity of proanthocyanidins on uropathogenic Escherichia coli in the prevention of recurrent urinary tract infections
<p>Abstract</p> <p>Background</p> <p><it>Escherichia coli</it>, the main bacteria found in recurrent urinary tract infections (UTI), is now frequently resistant to several currently used antibiotic treatments making new solutions essential. In this study, we evaluated the association propolis and proanthocyanidins type A to reduce bacterial anti-adhesion activity of <it>E. coli </it>on urothelial cells.</p> <p>Results</p> <p>This first double-blind, randomized, cross-over human trial included 5 volunteers that followed 6 different regimens with or without variable doses of cranberry and propolis with a washout period of at least 1 week between each regimen. Urine samples were collected at 0 h, 4-6 h, 12 h and 24 h after cranberry plus propolis or placebo capsule consumption. In vivo urinary bacterial anti-adhesion activity was assessed with a bioassay (a human T24 epithelial cell-line assay) and an in vivo <it>Caenorhabditis elegans </it>model. HPLC-PDA-MS was used to detect propolis and cranberry compounds in urine. Bioassays indicated significant bacterial anti-adhesion activity in urine collected from volunteers who had consumed cranberry plus propolis powder compared to placebo (<it>p </it>< 0.001). This inhibition was clearly dose-dependent, increasing with the amount of PACs and propolis equivalents consumed in each regimen. Results suggested that propolis had an additional effect with PACs and prevent a bacterial anti-adhesion effect over 1 day. An in vivo model showed that the <it>E. coli </it>strain presented a reduced ability to kill <it>C. elegans </it>after their growth in urine samples of patients who took cranberry plus propolis capsules. HPLC confirmed that propolis is excreted in urine.</p> <p>Conclusions</p> <p>This study presents an alternative to prevent recurrent UTI. Administration of PACs plus propolis once daily offers some protection against bacterial adhesion, bacterial multiplication and virulence in the urinary tract, representing an interesting new strategy to prevent recurrent UTI.</p
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